50 MOBPHOLOGY AND EVOLUTION In all Cercopithecidse the epidermis is thickened to form callosities overlying the ischial tuberosities; and the latter form broad supporting platforms for the callosities in the Drill and Mandrill. The head is rounded, and the nose is flattened, or prolonged to form a proboscis in Nasalis. The jaws exhibit a variable amount of prognathism; and the latter is most marked in the Cynocephali. The depth of the jaws below the orbits is greater than in the Cebidse, but the top of the cranium is more elevated above the orbits in the latter. The eyes may be relatively large or small, and are dark in colour in the Cercopitheques. In some forms, particularly the Baboons and some Macaques, they appear wicked and cunning. The nostrils are separated by a narrow partition, except in the Guerezas, and are directed downwards. The ears are reduced. Vibrissse.—The increased acuteness of the tactile sense in the hands has been accompanied by a reduction in the vibrissse. These hairs are only found in the supra-orbital, nasal and labial regions ; for the genal and submental tufts are entirely absent. Hair Slope.—Kidd has shown that the primitive condition is a general cranio-caudal slope of the hairs from the scalp right back to the tail, and downwards along the limbs. In the Cercopithecidse the primitive arrangement is maintained, but there is a convergence of streams towards the elbows as in the Anthropoid Apes. No spirals are present on the chest or abdomen as in the Lemuroidea and Tarsioidea. But there is a spiral on the vertex of the head. There are two pectoral mammae.